Intel Core 2 Duo Upgrade
Should I upgrade form an Intel Core 2 Duo e4500 to e6700?
^ Agreed. It's not enough of an upgrade to bother with. Although if you can get it for very cheap and it's one of the 3.2Ghz Wolfdale based Core2 chips and you have a motherboard capable of overclocking you can hit 4Ghz fairly easily with a good cooler. I would not spend very much on such a venture but if you can do all that cheap it might be a decent upgrade from a stock E4500.
Note that there are 2 chips called E6700. The first is a first generation 65nm Conroe core that is stock at 2.66Ghz. The newer 45nm Wolfdale core at 3.2Ghz is the one you want.
I realize that (well, assume really) you are trying to get a bit more life out of an older system with a few upgrades and spending as little cash as possible. But unfortunately, there comes a point where you need to decide whether it is worth it to spend money on upgrades, or would those dollars be better put towards a future replacement system. That is a decision that you will need to make on your own based on your personal circumstances.
When Tom's publishes their list of CPU's to buy, (eg. "Best Gaming CPUs For The Money: November 2013" http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106.html) on the last page they have a grid of where each CPU stands.
They generally recommend only upgrading if you can move up at least two tiers on the chart.